An ophthalmic lens is prepared for mounting in a rimmed, half-rimmed, or rimless (or “drilled”) eyeglass frame by acquiring the desired outline along which the ophthalmic lens is to be shaped so as to adapt it to the shape of the eyeglass frame. The outline is positioned on the lens as a function of the optical frame of reference of the lens so that, while it is being worn, the lens is suitably positioned in front of the wearer's eye. The lens is then shaped to match the desired outline.
During shaping, it can happen that certain portions of the desired outline cannot be shaped without the tool carrier or the tool coming into conflict with some other element of the machining device, e.g. the blocking support for supporting the lens in the machining device. This typically happens when the corresponding portion of the desired outline is situated close to the lens support or when the diameter of the tool is substantially smaller than the diameter of the tool carrier.
Ophthalmic lenses that are to be shaped with outlines of small height and large width, generally rectangular outlines, often give rise to this type of interference.
The same applies to ophthalmic lenses that are to be shaped with outlines presenting shapes that are complex, e.g. including zones curved in towards the center of the lens, referred to as zones of negative curvature. These zones of negative curvature generally correspond to decorative details in the outline of the lens and they need to be machined by a tool presenting a diameter that is smaller than the diameter of the grindwheel commonly used for shaping the lens. Under such circumstances, it is possible to use a cutter tool presenting a diameter of at few millimeters, for example.